1. Field of the Invention
The present invention generally relates to a technique for controlling an electric heater and, more particularly, to a heater control apparatus and method for controlling an electric heater provided in a fixation apparatus of an image forming apparatus such as a copy machine or a printer using an electrophotography method.
2. Description of the Related Art
An image forming apparatus using an electrophotography method, such as a copy machine or a printer, comprises: an image forming part which includes a photosensitive material and a electric charger part, an exposure part, a developer part, a transfer part, etc., provided around the photosensitive material; and a fixation device which fixes a toner image transferred onto a transfer paper by the transfer part. The fixation device is provided with a fixation roller having an electric heater therein. In order to keep the temperature of the fixation roller constant, also provided is a heater control device which controls electric supply to the heater.
Such kind of a heater control device is, disclosed in Japanese Laid-Open Patent, Application No. 2000-322137. The heater control device disclosed in this patent document comprises: a triac which turns on and off electric supply from a commercial power source to a heater; and a trigger means which turns on the triac at a predetermined timing with reference to a zero-cross signal output from a zero-cross detection circuit when a voltage of the alternating current of the commercial power source becomes 0 volt. The trigger means outputs a trigger signal at a predetermined timing (phase angle) on the basis of the above-mentioned zero-cross signal, and turns on the triac by the trigger signal. The triac maintains an on state until a subsequent zero-cross signal is output. Therefore, the conduction angle in a half cycle of the alternating current varies according to the timing of generation of the trigger signal. Thereby, an effective value of the alternating current (AC) voltage applied to the heater can be changed, and the electric power supplied to the heater can be controlled.
Since the power supply to the heater is controlled by a phase control of a conduction angle of the switching means such as a triac in the above-mentioned heater control device, information regarding power source frequency such as a zero-cross signal. However in Japan, for example, there are areas where the frequency of the alternating current of the commercial power source is either 50 Hz or 60 Hz. Even in such a case, it is desirable that the same apparatus can be used in either area regardless of the frequencies. For this reason, it is necessary to detect automatically the frequency of the alternating current supplied by the power source accurately, so as to determine the timing of turning on the triac within each half cycle in response to the frequency of the alternating current to generate the trigger signal and perform a phase control of the triac. Therefore, a power supply control for the heater cannot by started until the detection of the zero-cross signal is completed.
Additionally, the waveform of a power source voltage is not always a complete sinusoidal wave. That is, the waveform of a power source voltage may be distorted in a case where, for example, a machine consuming a large electric power is operated in a plant or the like located in the vicinity or a private power generation facility is used. Accordingly, the zero-cross signal is disturbed by the distortion of the waveform, which may prevent an accurate frequency detection. In such a case, the above-mentioned power supply control of the heater cannot be performed, which results in a detection of an abnormal condition and a display of an error display. Thus, the image forming apparatus may be set in an inoperative state.
Thus, in the technique disclosed in the above-mentioned patent document, a first trigger generating means and a second trigger generating means are provided, the first trigger generating means for generating a trigger signal in synchronization with the zero-cross signal and the second trigger generating means for generating a trigger signal in a synchronization with the zero-cross signal. The first trigger generating means is operated when a zero-cross signal of an alternating current supplied by a commercial power source is detectable, while the second trigger generating means is operated when the zero-cross signal is not detectable. Thereby, even when it is not possible to detect a zero-cross signal, electric power supplied to an object to be controlled such as a heater of a fixation device can be controlled.
However, the above-mentioned technique requires an increase in the manufacturing cost of the image forming apparatus since two kinds of trigger generating means are needed. In addition, an electric power control cannot be started until it is known as to whether a zero-cross signal is detectable by the zero-cross signal detecting means. Additionally, there is a problem in that an accurate control responsive to the power source frequency cannot be performed since the electric power supplied to the heater is controlled by performing a phase control of a triac using a trigger signal which is asynchronous to the zero-cross signal when it becomes impossible to detect the zero-cross signal.
Recently, a start-up time of fixation has been considered to be important as a specification when controlling a heater of a fixation device provided in an image forming apparatus. Therefore, even a time for detecting a power source frequency cannot be disregarded, and it is desirous to reduce such a time for detecting a power source frequency.